The present invention relates to a method and a circuit arrangement for boosting the pulses of video signals, for example in monochrome facsimile reproduction.
In a facsimile scanning arrangement, an original which is to be copied is scanned point-by-point and line-by-line by a scanning device having one or more optoelectronic transducers and the brightness information of the original is converted into a video signal.
The original may be a printed document or a document in typescript, a handwritten text or a graphic representation.
The video signal obtained by scanning the original is converted into a two-level monochrome signal by comparison with a threshold signal and transmitted to a receiving apparatus on a transmission channel. The recording device of the receiving apparatus, which device is controlled by the monochrome signal, records a facsimile of the original.
Due to the limited resolving power of the scanning device, when thin strokes and lines are scanned and also in the case of originals with a low contrast, only small, falsified signal amplitudes are produced and these often do not reach the threshold signal and therefore also do not produce any change in the monochrome signal. In this way, for example, thin black strokes on a white background or thin white strokes on a black background are lost in the facsimile record.
To avoid such losses of information, it is known to subject the video signal to double differentiation for the purpose of boosting its amplitude before it is compared with the threshold signal. The boosted video signal now interesects the threshold signal and the monochrome signal contains all the information required for faultless transmission.
Double differentiation can be carried out meaningfully, however, only if the video signal has a continuous-value curve, as is the case for instance in the scanning of originals with a discrete optoelectronic transducer.
In order to increase the scanning speed, however, it is customary to scan the original with a number of optoelectronic transducers (line of photodiodes) at the same time.
Such a line of photodiodes supplies, for example, a stepped, discontinuous-value video signal which is not suitable for double differentiation.
In addition, when a line of photodiodes is employed, there is the fact that as a result of the optical imaging of the information in the original on the line of photodiodes and of interference by adjacent photodiodes in the line, a flattened video signal apppears and, consequently, a loss of detail. This loss of detail can be compensated by suitable pulse boosting.
It is therefore an object of the invention to provide a method of boosting pulses in the case especially of a discontinuous-value vidoe signal, by means of which the quality of recording is improved.